Parvicapsula pseudobranchicola: life cycle and genetic variation

Parvicapsulosis caused by the myxozoan Parvicapsula pseudobranchicola has been a recurring problem in the Norwegian salmonid farming since 2002 with individual companies experiencing economic losses of several million NOK annually. The biology of this parasite is not well known, particularly the source (environmental reservoirs) of the infective stages. For unknown reasons, parvicapsulosis is a problem mostly affecting northern Norway. Therefore, the project had two main objectives; 1) Identify the unknown annelid host(s) of the parasite representing the source of waterborne infective spores, and 2) Develop molecular markers in order to study the genetic variation in P. pseudobranchicola. This aims at producing epizootiological knowledge that eventually could be used to reduce losses due to parvicapsulosis. The final host of P. pseudobranchicola was not identified in the project although approximately 2000 polychaetes of different species from different farming sites and water depths were analysed. Several species were examined in high numbers and are now considered unlikely as potential final hosts. In collaboration with University of Bergen and Mainstream AS, wild salmon (Salmo salar) and sea trout (Salmo trutta) were analysed for presence of P. pseudobranchicola by real-time PCR. For salmon a decreasing trend in the prevalence of infection from the north to south was seen, but for trout there was no clear trend. Ribosomal 18S, ITS1 and ITS2 sequences were recovered from geographically widespread isolates from salmon, sea trout and Arctic charr (Salvelinus alpinus), but proved to be identical across the host and geographic range examined. Variation in ITS was expected on the basis of other studies on myxosporea in salmonids. A lack of genetic variation indicates that strain differences is a less likely explanation for the particular parvicapsulosis problems in Northern Norway. Infections in salmon stocked in spring and autumn were followed on a weekly basis and it was found that it takes approximately a month longer before fish stocked in spring are infected compared to fish stocked in autumn. The latter group becomes infected almost immediately after stocking. The results suggest that infective spores are not present in the water before June/July and this has implication for management on fish farms.

The myxozoan parasite Parvicapsula pseudobranchicola were first reported from seawater reared Atlantic salmon in Norway in 2002 as the cause of severe disease and economical losses and has since then been an increasing problem in the Norwegian aquaculture industry. Individual farms experience economical losses in the range of 10 millon NOK annually. Myxozoans have complex life cycles usually involving an intermediate fish host and an annelid main host. More than 2200 myxozoan species have been described, however, less than 30 life cycles are known. From the marine environment 4 myxozoan life cycle have been described, all use polychaetes as main host. To reduce infection pressure by myxozoans in farmed fish, knowledge about the life cycle is essential. We will therefore try to identify the the main host of P. pseudobranchicola to enable laboratory experiments to study factors that will influence the life cycle of the parasite such as: * host preference, suitability of different species of fish * temperat ure effect on the release of infective spores * susceptibility of different host strains and age-groups * spores release in relation to seasonality * spore viability , e.g. longevity in seawater Furthermore, most cases of parvicapsulosis are from the no rthern parts of Norway even though the parasite have been found as far south as the Oslofjord. The reason for the observed increase in the number of cases of parvicapsulosis in northern parts of Norway, compared to other regions with high densities of fis h farms in Norway, may be due to strain variations in P. pseudobranchicola. New molecular markers to study the genetic variation in P. pseudobranchicola will therefore be developed.